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PRACTICAL  ORGANIC  CHEMISTRY

burners are therefore not lighted under the boat until towards
the close of the combustion. In the case of a highly volatile
compound like ether, a combustion tube is used, which projects
at least 15 cm. (6 in.) beyond the back of the furnace. The
bulb containing the substance is then placed just outside the
furnace, and then the spiral in contact with it. A small Bimsen
flame is placed under the end of the spiral away from the sub-
stance, the heat from which is sufficient to completely volatilise
the substance at a convenient speed.
The Combustion of Organic Substances containing-
Nitrogen.—The following modification must be introduced in
cases where the organic substances contain nitrogen. As the
nitrogen may be liberated in the form of one or oilier of its
oxides, which are liable to be absorbed in the potash apparatus,
a source of error is introduced, which may be eliminated in the
following way. A spiral of metallic copper is brought into the
front end of the combustion tube, which, when red hot, reduces
the oxides of nitrogen. The free nitrogen then passes through
unabsorbed. About 13 to 15 cm. (5 to 6 in.) of coarse
copper oxide is removed from the front end of the tube, and
after inserting an asbestos plug, the space left by the oxide 5s
filled with a roll of copper gauze 13 to 15 cm. (5 to 6 in.)
long. The copper spiral must have a clean metallic surface,
which is easily produced in the following way. Take a large
test-tube or boiling tube, an inch or so longer than the spiral,
and push down to the bottom a small pad of asbestos. I'our in
about 5 c.c. of pure methyl alcohol.
Have a cork at hand which fits loosely into the mouth of the
test-tube. Wrap the tube round with a duster. Hold the cop-
per spiral with the crucible tongs in a large blow-pipe flame until
it is red hot throughout and slide it quickly into the test-tube.
The methyl alcohol reduces the film of oxide on the copper ami
is at the same time oxidised to formaldehyde, the vapours of
which attack the eyes if the tube is brought too near the fare.
The alcohol takes fire at the mouth of the test-tube. When the
flame dies down insert the loose cork and let the tube cool. The
spiral, which has now a bright surface, is withdrawn, and tin-
excess of alcohol removed by shaking it. It must now be
thoroughly dried. Place the spiral in a hard glass tube a feu-
inches longer than the spiral and fitted at each end with a cork,